Field of the Invention
The invention relates to a safety control system for protecting hydraulically held loads against uncontrolled pressure over-loading, for example, in hydraulic cylinders of vertical presses or like load holding devices.
The press ram of a vertical press is commonly hydraulically returned to its top starting position by means of retraction cylinders after the press stroke is completed. For safety reasons, these retraction cylinders, in which slide pistons or plungers which are connected via tie rods to the ram, are provided with openable non-return valves which are adapted to close instantly in the event of failure of breakage of the working pressure duct by way of which the retraction cylinders are pressurised, so that the load of the ram is held and it is possible to avoid endangering personnel or destroying the press.
On the other hand, the retraction cylinder or cylinders must be open to the fluid discharge or tank during the descent of the main press piston connected to the ram, i.e., the controlled non-return valves which are disposed on the retraction cylinder must be opened by control pressure so that the hydraulic fluid in the retraction cylinder can be discharged therefrom by the descending piston. If the controlled non-return valves do not open there is a risk of bursting of the cylinders. During the press stroke the pump applies fluid pressure to the main press cylinder whose piston surface area can amount to approximately six times the piston surface area of the retraction piston. In the non-return valves do not open, the pressure applied to the piston surfaces of the retraction pistons will be six times that of the main press piston. The pistons however cannot yield. Destruction of the cylinders with all the attendant risks for the personnel would be the consequence.